The present invention relates to a sheet post-processing unit such as a sorter for distributing and accommodating printed sheets discharged from an image forming machine, and to an image forming apparatus having the sheet post processing unit and an image forming machine such as a copying machine, a printer and the like for forming images on a printing sheet.
Various types of sheet post-processing units for distributing printed papers which have images formed thereon and discharged from a stencil printing machine have heretofore been developed and put to practical use. Sheet post-processing units of the sorts mentioned above are desired to be devised so that while every possible effort is made for size reduction, a larger number of printed sheets may be distributable.
In such a sheet post-processing unit of a fixed bin type, a plurality of bins are fixedly arranged in the vertical direction of a casing and conveyer units having fans and blowers are installed in the vertical direction of the plurality of bins and besides indexers as sheet guide means for carrying printed papers into the corresponding bins are moved up and down vertically along travel routes of the conveyer units. Therefore, an attempt has been made to reduce the size of the whole apparatus by decreasing the dimension thereof in the depth direction. With respect to the conveyer unit, moreover, the diameter of a corner portion at both ends of a belt has also been designed for its size to be minimized. However, the following problems still exist because various types of paper are used as printing sheet in the stencil printing machine and aforementioned paper post-processing unit.
In the sheet post-processing unit thus arranged as described above, an indexer 116 is moved up or down to the position of a target bin 78 and the printed sheet P conveyed from a conveyer unit 92 is peeled off the belt surface at the indexer 116, guided to and accommodated in the target bin 78 as shown in FIG. 12. When the printed sheet P is carried into each bin 78 from the indexer 116, however, not only a paper jam percentage but also accommodating capacity per bin may greatly vary if the stop position of the indexer 116 is unchanged even in a case where the paper thickness varies.
More specifically, no problem will particularly be caused even though the stop position of the indexer with respect to the bin 78 is left unchanged in a case where the printed sheet P conveyed from the conveyer unit 92 is ordinary paper. Nevertheless, the thick printed paper P conveyed from the conveyer unit 92 for next distribution becomes unaccommodated in the target bin 78 because it is blocked by sheets of printed paper P that have already been stacked up therein as the number of printed sheets per bin increases. Consequently, the paper accommodating capacity per bin tends to decrease.
In the sheet post-processing unit thus arranged as described above, further, a corner portion in the travel route of the conveyer unit 92 is considerably curved and therefore one end of printed sheet tends to become easily curled in the corner portion of the belt, depending on the type of sheet (see FIG. 12). Therefore, as in the case where thicker printed papers are distributed, a paper jam is readily produced in the proximity of the entrance of the bin 78 with respect to the indexer 116 since the printed paper conveyed from the conveyer unit 92 for next distribution becomes unaccommodated in the target bin 78 as it is blocked by the sheets of printed paper P that have already been stacked up therein.
When a large number of thick printed papers are accommodated in one bin or when the end of that type of paper becomes curled in the travel route, printed paper for next distribution becomes unaccommodated in the target bin as it is blocked by the sheets of printed paper P that have already been stacked up therein, which results in not only decreasing the paper accommodating capacity but also causing a paper jam.